Direct access data storage devices are well known in the art, thus no attempt will be made to describe these devices in detail. It suffices to say that in such devices, and particularly in magnetic disk devices, the ability to properly record or write certain stress sequences of binary "1's" and/or "0's" is a function of the disk-to-head interface and the magnetic recording phenomenon. When an attempt is made to record such stress patterns, the linear track position of the recorded data (i.e. the S-S or N-N magnetic transitions along the track) may shift from the desired position, and as a result, subsequent reading of the data may produce a read-error(s). In addition, it is known that error correction codes are vulnerable to certain data patterns that stress the ability of the ECC to identify and/or correct read-errors. One example of a read stress pattern is a one reoccurring sequence of equal (i.e. repeating) magnitude read-errors.
The present invention relates to the field of DASD, and provides a scheme for randomizing binary data prior to recording, and for derandomizing the randomized data in a complementary (i.e. similar) fashion upon read back, such that write-patterns that stress the head/media interface are avoided, and such that read-patterns that stress the ECC are avoided.
To the knowledge of the present inventors, the concept of randomizing binary data prior to storing the data on disk storage devices, and for the purpose of avoiding data patterns that stress the head/disk interface, is not to be found in the prior art. In addition, it is believed that the concept of, at a later and a random time (i.e. not real time, as in a data transmission system), reading back the randomized data from the disk, and then derandomizing the data prior to presenting the derandomized data to an ECC network, is not to be found in the prior art.
The use of scrambler/descrambler schemes in real time transmission path digital communication systems is known. U.S. Pat. No. 4,639,548 is an example. In this patent, an input data signal is scrambled and then applied to a convolutional encoder. The output of the encoder is then applied to a transmission path. The data transmitted through the transmission path to a receiver is applied first to a convolutional encoder, and then to a descrambler. The descrambler is synchronized to the scrambler by the use of an error correction pulse signal that is generated at the location of the descrambler.
U.S. Pat. No. 3,775,746 also describes the use of a scrambler/descrambler scheme in connection with a real time digital data transmission system. In this system, the scrambler/descrambler function is provided by a network that includes a delay circuit and an exclusive-or gate. Synchronization and clocking are not described in this patent.
U.S. Pat. 3,771,126 describes a data processing and transmission system wherein an analog signal is sampled and encoded by an encoder. A parity generator then adds parity bits to allow for the correction of errors in each block of digital data. The digital data and the parity bits are then converted into a random sequence of pulses by a scrambler, prior to being applied, real time, to a channel. At the output of the channel, the data is descrambled, applied to an error decoder, and then reconstructed into an analog signal, of for use by a utilization circuit. The scrambler is said to be a conventional self-synchronizing scrambler, for example one cell scrambler/descrambler comprising a modulo 2 adder and a one cell delay.
U.S. Pat. 3,988,538 describes scramblers and descramblers for use in digital data transmission systems having a plurality of parallel synchronized data streams. The complementary scramblers/descramblers of this patent are each made up of a series of delay elements (i.e. a shift register) comprising a linear sequential filter having a series of taps (i.e. a tapped delay line) that are summed and then connected in feedback/feedforward fashion to the input of the scrambler/descrambler.
These patents are exemplary of the prior art in which data is scrambled for transmission in real time to a receiver having a descrambler.
While the prior art is useful for the purpose intended, it does not suggest the use of a complementary randomizer/derandomizer combination in the write/read channels of a DASD device. In accordance with the present invention, randomized data is recorded or stored on a non volatile medium such as a magnetic disk. The DASD device is then used to read back the randomized data to a read channel having a complementary derandomizer. The DASD randomizer/derandomizer combination functions to avoid the writing of data patterns that stress the ability to store the randomized data, and also functions to avoid read-error conditions that stress the ability of the read channel's error correcting code (ECC) to identify and correct errors that may occur when the stored, randomized, data is subsequently read back from the disk.